1. Field of the Invention
One disclosed aspect of the embodiments relates to distributed feedback surface emitting lasers that use photonic crystals.
2. Description of the Related Art
In recent years, there have been many reports that discuss examples of semiconductor lasers to which photonic crystals are applied. Japanese Patent Laid-Open No. 2000-332351 discloses a surface emitting laser in which a two-dimensional photonic crystal (two-dimensional diffraction grating) is disposed near an active layer formed on a substrate and containing a light-emitting material. This is a type of a distributed feedback surface emitting laser (DFB surface emitting laser). The two-dimensional photonic crystal is periodically provided with cylindrical air holes in a semiconductor layer and has two-dimensionally periodical refractive-index distribution.
Light generated in the active layer is second-order diffracted due to the periodical refractive-index distribution in the photonic crystal while being guided in an in-plane direction of the active layer so as to form a standing wave, which is a resonant mode, at a specific wavelength λ, thereby causing laser oscillation. This laser-oscillated light undergoes first-order diffraction by the photonic crystal so as to change its propagation direction to a direction perpendicular to the plane of the active layer before the light is output from the surface of the laser structure.
Since this phenomenon occurs in the entire area of the photonic crystal, the semiconductor laser discussed in Japanese Patent Laid-Open No. 2000-332351 operates as a surface emitting laser that outputs two-dimensional coherent light. One characteristic feature of this laser structure is that the direction in which the light is guided near the active layer and the direction in which the light is output from the laser structure are perpendicular to each other.
In addition to this laser structure, a surface emitting laser structure that uses a photonic crystal in a vertical cavity surface emitting laser (VCSEL) is also known. Japanese Patent Laid-Open No. 10-284806 discloses a VCSEL that suppresses spontaneous emission of light by using a two-dimensional photonic crystal so as to allow for low power consumption operation with a low threshold value.
Unlike the aforementioned DFB-type photonic crystal surface emitting laser, the direction in which the light is guided near the active layer and the direction in which the light is output from the laser structure are the same in this VSCEL-type photonic crystal surface emitting laser.
The term “photonic crystal surface emitting laser” used in this specification will be directed to a distributed feedback (DFB) surface emitting laser, unless otherwise noted.
As mentioned above, in a photonic crystal surface emitting laser, the resonating direction of light is oriented in the in-plane direction of the active layer. If the size of the photonic crystal in the in-plane direction of the active layer is limited, a phenomenon in which a portion of resonating light leaks out from the photonic crystal occurs. If multiple photonic crystal surface emitting lasers are arrayed on a single substrate, light leaking from one of neighboring photonic crystal surface emitting lasers may reach the other photonic crystal surface emitting laser, possibly having an adverse effect on the oscillation characteristics.
Japanese Patent Laid-Open No. 2010-093127 discloses a semiconductor light-emitting device in which multiple photonic crystal surface emitting lasers are arrayed on a single substrate. In this device, grooves are formed between the photonic crystal surface emitting lasers, and the surface of each groove is coated with a reflective film. With this configuration, input of leaking light from one photonic crystal surface emitting laser to another photonic crystal surface emitting laser may be suppressed, thereby achieving stable oscillation characteristics.